Wednesday, November 15, 2006 - 12:25 PM

Fifty Years of Predicting Wheat Nitrogen Requirements Based on Soil Water, Yield, Protein and Nitrogen Efficiencies.

William Pan1, William Schillinger1, David Huggins2, Richard Koenig1, and John Burns1. (1) Washington State Univ, Dept of Crop & Soil Sciences, PO Box 646420, Pullman, WA 99164-6420, (2) USDA-ARS, Washington State Univ, Pullman, WA 99164-6420

During the early 1950’s synthetic N fertilizers were gaining widespread adoption in the wheat growing region of the inland Pacfic Northwestern U.S.   Agronomists quickly recognized water and N as the two principal determinants of grain yield and quality  Numerous N fertility trials across a range of climatic environments, soils and cropping systems provided the basis for estimating wheat yield potentials from root zone soil moisture, and N fertilizer recommendations were made from yield-based crop N requirements, estimates of soil N supplies and N use efficiencies.  This N recommendation model based on the regional variations in crop-soil N budgets (Leggett, 1959) has stood the test of time for nearly 50 years, as confirmed by recent N fertility and agronomic trials.   A recent data analysis of yield-water relationships by Schillinger et al. reveals a remarkably similar slope but different y-intercept defining the lowest available water levels at which grain yields are obtainable.  Spring soil moisture remains a reasonable predictor in this Mediterranean climate, but variable in-season rainfall is still a major source of error.  Adjustments in the N recommendation model have been made to accommodate genetic, soil, management and climatic variables affecting water and N use efficiencies.  However, limits in our ability to extrapolate the regional model to site specific applications are restricted by our ability to predict landscape processes that control the water-yield and the yield-nitrogen use relationships defining the unit N requirement. The generalized 50% single season N uptake efficiency used in the model is adjustable with improved N management, but much higher rotational N uptake efficiencies need to be recognized and taken into account.